2. Field of the Invention
The present invention relates to an apparatus for storing heat energy, which converts heat energy into chemical energy to store it, and which takes out heat energy when necessary, as heat or cold. Particularly, the invention relates to an apparatus for storing heat energy by using a liquid heat-storing medium.
2. Description of the Prior Art
To effectively utilize energy, attempts have heretofore been made to use natural energy such as solar energy and to use low-cost energy such as waste heat from the factories. However, the supply of heat of this sort is low in density and unreliable. Therefore, it is necessary to accumulate and store the low density energy so that it can be utilized.
With an air-conditioning apparatus which utilizes heat of the above-mentioned, it is desired to temporarily store heat and to take out heat as required. Here, heat energy to be taken out should produce a temperature as low as possible when cooling is to be effected and should produce a temperature as high as possible when heating is to be effected.
To store heat energy in the form of thermal motion includes (1) a method of utilizing the sensible heat (temperature difference), and (2) a method of utilizing the latent heat (phase transformation from solid to liquid, or from liquid to gas). To store the heat energy by converting it into chemical energy includes (3) a method of utilizing the heat of chemical reaction, (4) a method of utilizing the difference in concentration to obtain the heat of dilution from a concentrated liquid, and a combination thereof.
Among these methods, a heat-accumulating system which uses water or the like having a high specific heat to utilize sensible heat, has a low heat-accumulating density (amount of heat accumulated per unit weight of the heat-accumulating liquid). Therefore, a large heat accumulator must be employed with heat-insulating construction having high heat retaining property, resulting in increased cost of manufacturing the apparatus. With a system which utilizes the heat of fusion of a solid material, the heat-accumulating density may be large. In the phase transformation from liquid into solid, the liquid is difficult to solidify due to supercooling, and is difficult to liquefy due to superheating. Moreover, the solid conducts the heat poorly, which makes the apparatus bulky.
With a system which stores energy in the form of thermal motion, the temperature level in principle is the same between when the heat is to be stored and when the heat is to be taken out. In practice, therefore, the temperature level decreases when the heat is taken out compared with when the heat is stored due to radiation of heat, temperature drop in exchanging heat and the like. At the time of utilizing heat, it is not allowed to change the temperature to meet the purpose. Besides, to accumulate heat, it is necessary to employ a heat-insulating construction having high heat retaining property to prevent the heat from being radiated.
According to the system which accumulates heat by converting heat energy into chemical energy, the energy can be stored at a relatively low temperature. Therefore, energy can be stored while greatly preventing the heat from radiating; i.e., energy can be stored for extended periods of time, presenting an advantage compared with the systems which utilize sensible heat or latent heat.
Japanese Patent Laid-Open No. 157995/1982 discloses a heat accumulator which employs a hydrophobic porous material. This is an apparatus which converts the heat energy into chemical energy to store it by utilizing heat of formation of a compound and heat of decomposition. In a reaction vessel in which the compound is formed or decomposed, there is provided a heat exchanger to exchange the heat produced by the reaction of the absorbing agent (NaI) with the coolant (NH.sub.3 gas) or the heat of decomposition of NaI.nNH.sub.3. A gas chamber for a coolant gas and an absorbing agent (NaI) reservoir are separated by a wall composed of a water-repellent porous material which permits the coolant gas to pass through but which does not permit an absorbing agent (NaI) to pass through based upon the water-repelling action. A reservoir vessel for storing the coolant (liquid NH.sub.3) accommodates a heat exchanger which exchanges the heat of condensation of the coolant and the heat of vaporization between the coolant and the absorbing agent, and has a heat-exchanging coil that surrounds the reservoir vessel to heat the coolant. This method does not require the broad space for spraying the liquid that was necessary in the conventional art, and enables the heat exchanger and the absorbing agent to be contained in a unitary form. However, the porous material is not allowed to be utilized on the side of the coolant because of the reasons mentioned below, and it is difficult to uniformly vaporize the coolant, imposing limitations on reducing the size of the apparatus. Furthermore, it is not allowed to utilize the porous material on the side of the coolant since it is not capable of preventing the droplets from spraying when the coolant is generated.